drm/vc4: hdmi: Check the HSM rate at runtime_resume

If our HSM clock has not been properly initialized, any register access
will silently lock up the system.

Let's check that this can't happen by adding a check for the rate before
any register access, and error out otherwise.

Link: https://lore.kernel.org/dri-devel/20220922145448.w3xfywkn5ecak2et@pengutronix.de/Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220929-rpi-pi3-unplugged-fixes-v1-2-cd22e962296c@cerno.tech

drivers/gpu/drm/vc4/vc4_hdmi.c

@@ -2869,6 +2869,7 @@ static int vc4_hdmi_runtime_resume(struct device *dev)
 	struct vc4_hdmi *vc4_hdmi = dev_get_drvdata(dev);
 	unsigned long __maybe_unused flags;
 	u32 __maybe_unused value;
+	unsigned long rate;
 	int ret;
 
 	/*
@@ -2884,6 +2885,21 @@ static int vc4_hdmi_runtime_resume(struct device *dev)
 	if (ret)
 		return ret;
 
+	/*
+	 * Whenever the RaspberryPi boots without an HDMI monitor
+	 * plugged in, the firmware won't have initialized the HSM clock
+	 * rate and it will be reported as 0.
+	 *
+	 * If we try to access a register of the controller in such a
+	 * case, it will lead to a silent CPU stall. Let's make sure we
+	 * prevent such a case.
+	 */
+	rate = clk_get_rate(vc4_hdmi->hsm_clock);
+	if (!rate) {
+		ret = -EINVAL;
+		goto err_disable_clk;
+	}
+
 	if (vc4_hdmi->variant->reset)
 		vc4_hdmi->variant->reset(vc4_hdmi);
 
@@ -2905,6 +2921,10 @@ static int vc4_hdmi_runtime_resume(struct device *dev)
 #endif
 
 	return 0;
+
+err_disable_clk:
+	clk_disable_unprepare(vc4_hdmi->hsm_clock);
+	return ret;
 }
 
 static int vc4_hdmi_bind(struct device *dev, struct device *master, void *data)